As is known, the filtration of fuel or lubricating oil is generally obtained with a filter comprising an external casing provided with an inlet for fluid to be filtered and a outlet for the filtered fluid, and a filter cartridge destined to sub-divide the internal volume of the casing into two distinct chambers, of which a first chamber communicates with the inlet and a second chamber communicates with the outlet.
In this way, the fluid which flows from the inlet towards the outlet of the filter is forced to cross the filter wall which retains the impurities that may be present therein.
A typical filter cartridge comprises a tubular filter wall, for example a pleated filter wall or a depth wall, and two support plates which are fixed to opposite ends of the filter wall.
At least one of the support plates is commonly provided with a central opening, aligned with the longitudinal axis of the filter wall, across which the internal volume of the filter wall is set in communication with the inlet of the fluid to be filtered, in a case in which the filter wall is configured such as to be crossed from inside towards the outside, or with the outlet of the filtered fluid, in a case in which the filter wall is configured such as to be crossed from outside towards the inside.
During use, it can happen that the fluid to be filtered encounters excessive hydraulic resistance to passage across the filter wall, which excessively reduces the inflow of the fluid to the devices located downstream of the filter.
This can happen for example when the quantity of impurities accumulated have reached the point of completely clogging the filter wall, or when, due to particular operating conditions, the fluid to be filtered reaches very high levels of viscosity, as happens in a case of diesel fuel filtration in diesel engines, when the operating temperatures are very low.
To obviate this problem, filters have been fitted with an automatic bypass valve which following an increase in pressure in the chamber of the fluid to be filtered, which pressure goes beyond a predetermined value, is destined to open a direct passage between the chamber of the fluid to be filtered and the chamber of the filtered fluid, thus by-passing the filter wall.
The prior art provides various types of bypass valves for filters, some of which are associated to the external casing of the filter, while others are directly associated to the filter cartridge.
The by-pass valves associated to the filter cartridge are normally positioned or fashioned in the centre of one of the support plates, substantially aligned with the longitudinal axis of the relative filter wall.
Although this arrangement facilitates the realisation of the by-pass valve, it considerably limits the design of the filter cartridge, as the central zone of the support plates is that which generally also bears the engaging means with the filter casing as well as being the zone which exhibits the openings destined to place the internal volume of the filter wall in communication with the inlet or the outlet of the filter.
Patent application no. WO 2010/129450 describes a filter cartridge having a support plate in which a bypass valve is incorporated, which is arranged in an eccentric position with respect to the longitudinal axis of the filter wall. This arrangement of the bypass valve has the advantage of not interfering with the engaging means of the cartridge. However, the solution proposed in this patent application has the bypass valve completely integrated in the support plate, i.e. the support plate is conformed such as to make available a direct passage between the chamber of the fluid to be filtered and the chamber of the filtered fluid, as well as a cylindrical seating suitable for directly accommodating a mobile obturator element and a spring suitable for pushing the obturator element towards a closed position of the passage.
Though effective from the functional point of view, this constructional form introduces some important drawbacks. A first drawback is that in a case of malfunctioning of the bypass valve, it is necessary to demount and replace the whole support plate, by means of an operation which is often slow, laborious and therefore rather expensive. A further drawback is that the bypass valve has also to be assembled together with the filter cartridge, which requires adopting mounting procedures which are dedicated for each type of filter cartridge produced, and thus reducing the possibility of obtaining advantageous economies of scale.